1. Field of the Invention
The present invention relates to the field of optical devices and more particularly to a light valve for amplifying light.
2. Related Application
The present application relates to copending application Ser. No. 07/497,134, filed Mar. 20, 1990, and titled "High Resolution Light Valve."
3. Prior Art
Various schemes for amplifying light are well known in the prior art. Devices which amplify light have been known since the late nineteenth century and these devices which amplify light on an area basis are commonly termed light valves. For example, U.S. Pat. No. 2,892,380 issued to Baumann et al. describes an arrangement of layers for providing an apparatus to amplify light from an optically projected image. In the Baumann et al. '380 patent, an apparatus is described in which a glass carrier has applied thereto an electrode of a transparent electrically conducting thin metallic layer. On this electrode there is a photoconductive layer of suitable thickness applied. On the photoconductive layer, a thin light-absorbing and electrically insulating layer having in turn an electrically non-conductive mirror layer is applied thereto. Over the mirror layer is a layer of a substance having an electric Kerr effect in which this layer has an optimum index of refraction which is variable when subjected to a variable electric field. Over this layer is a glass plate provided with an electric layer. Thus the mirror layer is located between the photoconductive layer to which the controlling light is applied and the layer with the electric Kerr effect through which the control light from the light source passes to the mirror and is reflected by the same through the layer with the electric Kerr effect. This arrangement described in the Baumann et al. '380 patent provides a basis for describing the essential features of current day light valves.
U.S. Pat. No. 3,592,527 (Conners et al.) describes a display device utilizing a nematic liquid crystal layer for displaying a projected radiation pattern by means of reflected ambient light. Subsequent use of light valves is described in U.S. Pat. No. 3,824,002 issued to Beard, in which an improved photo-activated liquid crystal light valve is described. In the Beard et al. '002 patent the electrically conductive elements of the light valve are separated from the liquid crystal layer by various insulating layers.
A different configuration is described in a patent application Ser. No. 538,381, filed Jan. 6, 1975, by W. P. Bleha, Jr., et al. and now abandoned, in which a liquid crystal material is in contact with inert insulating layers. In response to an external writing light, the photo-responsive layer modulates the field impressed upon the liquid crystal which, in turn, effects the birefringent optical activity of the liquid crystal. This change in birefringence is used to change the polarization state of linearly (or circularly) polarized read-out light. The Bleha et al. application further describes the response characteristics of a birefringent liquid crystal which represent respectively the transmission of the light valve for each of the three primary colors. By the application of proper voltage, a color projection capability is obtained from the scheme of Bleha et al. Further, an electro-optical system for real-time projection of very high brightness, full color pictorial images from a program source onto a large display screen from a set of three reflection type AC driven field effect mode birefringent liquid crystal light valves is described in U.S. Pat. No. 4,127,322, issued to Jacobson et al.
Further, in U.S. Pat. No. 4,019,807 (Boswell et al.), a reflective liquid crystal light valve with hybrid field effect mode is described. The liquid crystal of the Boswell et al. '807 patent is operated in a hybrid field effect mode and it utilizes the twisted nematic effect to create a dark off-state and the optical birefringence effect to create the bright on-state. A liquid crystal thus modulates the polarization of the coherent read-out or projection light responsively to the non-coherent image.
As is apparent from the description above, various light valve designs are known in the prior art. However, the use of prior art light valves in a video projection device has severe restrictions in that prior art light valves have switching characteristics which are severely limited. For example, in a video projection device which is to display motion, an image is typically changed 24 times per second in order for the human eye to satisfactorily perceive motion. Because of the limited switching characteristics of prior art light valves, resolution and performance of light valve amplification is severely limited and curtail the available grey scale rendition.
Accordingly, it is appreciated that what is needed is a light valve for amplifying light but having faster switching characteristics in order to provide a higher performance over prior art light valves in order to improve the display of motion in a video projection device.